There are a variety of types and sizes of fuses which are presently employed in different electrical and electronic circuits, and, indeed, their use in such circuits has been known for years. As it is well known, a fuse is a device intended to melt and open an electrical circuit whenever the ampere load on the circuit exceeds a predetermined safe value, i.e., the rated current capacity of the fuse. However, in some circuits such as, for example in A-C motor circuits, the fuse opens too quickly on moderate overloads. In order to overcome this difficulty, socalled time-delay (time-lag) fuses are employed which open the circuit only after an overload period of several times as long as that of an ordinary fuse.
Fuses having a fusible wire element wound over a core member made of aluminum oxide (alumina; Al.sub.2 O.sub.3) and magnesium oxide (magnesia; MgO) have been used in the past. The core member of this type of fuse usually have a star-shaped or irregular cross-section and includes a means for interrupting the electric arc which is placed in the fuse. These fuses however are designed for rapid cooling of the heat generated by the electric current by utilizing the high heat conductivity and high heat diffusivity of alumina and magnesia from which the core member is made. However, these fuses are not intended to be used as time-lag fuses since they do not possess time lag charcteristics but rather, they are used whenever improved rated current capacity is needed.
Spring type fuses having time lag characteristics have also been in general use. These types of fuses which employ low melting point solder as their heat storage element have been difficult to mass produce while maintaining a fixed tensile strength on the spring and an adequate amount of low melting point solder. Additionally, they have the inherent defect of straggling in their fusing characteristics due to the heating action arising from repeated current loads during use or long term adverse effect on the spring tensile force.
Another type of time-delay fuse employs a single fusible wire element wound over a glass fiber or a glass tube. However, since glass has a low softening point (650.degree.-700.degree. C.), and it is necessary to use a wire having a lower melting point than the softening temperature of the glass, this limits the types of wires that can be employed in this type of fuse.
Other time-delay types of fuses are also known, but none of these prior art types of fuses have proven to be entirely satisfactory for one reason or another as will become more evident from the ensuing description of the invention.